This application relates to coated articles, and, more particularly, to a superalloy article having a metallic overlay protective coating.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. However, the maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to 1900-2100F.
Many approaches have been used to increase the operating temperature limit of the turbine blades and vanes to their current levels. The composition and processing of the materials themselves have been improved, and physical cooling techniques are employed.
In another approach, a protective layer or a ceramic/metal thermal barrier coating (TBC) system is applied to the turbine blade or turbine vane component, which acts as a substrate. The protective layer with no overlying ceramic layer (in which case the protective layer is termed an xe2x80x9cenvironmental coatingxe2x80x9d) is useful in intermediate-temperature applications. The currently known protective layers include diffusion aluminides and MCrAlY(X) overlays.
A ceramic thermal barrier coating layer may be applied overlying the protective layer, to form a thermal barrier coating system (in which case the protective layer is termed a xe2x80x9cbond coatxe2x80x9d). The thermal barrier coating system is useful in higher-temperature applications. The ceramic thermal barrier coating insulates the component from the combustion gas, permitting the combustion gas to be hotter than would otherwise be possible with the particular material and fabrication process of the substrate.
Although these coating systems are operable, there is always the need to achieve further improvements in maximum operating temperatures and times of coated articles. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for preparing a coated article protected by a protective layer, and the article itself. The protective layer is suitable for use as an environmental coating with no overlying thermal barrier coating, or as the bond coat for a thermal barrier coating. The elevated temperature oxidation performance of the coated article is improved over that of conventional coated articles.
A coated article comprises an article substrate having a free sulfur content of more than 0 but less than about 1 part per million by weight (ppmw), and a protective layer at a surface of the article substrate. The article substrate is preferably a nickel-base superalloy, in the shape of a component of a gas turbine aircraft engine such as a turbine blade or turbine vane. The protective layer comprises a platinum aluminide diffusion coating. A thermal barrier coating layer made of a ceramic such as yttria-stabilized zirconia may overlie the protective layer, which is then termed a bond coat. There may instead be no overlying ceramic thermal barrier coating layer, in which case the protective layer is termed an environmental coating. Both the protective layer and the ceramic thermal barrier coating layer, where present, are also low in sulfur, preferably less than about 1 ppm by weight.
The protective layer may be substantially yttrium free, with less than about 10 parts per million. The protective layer may instead contain a substantial amount of yttrium, typically from about 10 to about 200 parts per million, for other applications.
The substrate article with low free sulfur content may be furnished in a variety of ways. The base metal may be selected to have a low free sulfur content. The composition of the base metal may be modified to result in a low free sulfur content. In one approach, a sulfur-scavenging element such as hafnium or zirconium is provided in the base metal in an amount sufficient to reduce the free sulfur content to less than about 1 part per million (ppm) by weight. In another approach, a conventional high-sulfur base metal can be provided. The base metal is contacted to a reducing gas to remove sulfur and reduce the free sulfur to the required low level. For example, the base metal can be contacted at elevated temperatures to hydrogen or a hydrogen-containing gas that desulfurizes the metal. In yet another approach, the molten base metal may be placed into contact with a reactive element such as calcium or magnesium, to react with and reduce the free sulfur content.
In the past, in many instances the sulfur content of the underlying substrate upon which the protective layer is deposited has not been reported. It may not be concluded from the absence of reporting of the sulfur content that the sulfur content is zero or otherwise less than about 1 part per million. Instead, in such situations it may be concluded that the sulfur content is likely in the typical range of about 5 to about 30 parts per million by weight, and that the sulfur content was not reported because there was no realization of its significance at the time.
The coated article of this type can be used in high-temperature applications in severe environments. A preferred application is as a gas turbine blade or vane, but the invention is not so limited. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.